A conventional optical communication link includes at least one optical transmitter at a first end of the link, such as a laser, one optical receiver at a second end of the link, such as a photodiode, and at least one optical waveguide interconnecting the transmitter and receiver. Such optical communication links are in wide use in today's data centers and supercomputers.
The desire for increased data throughput in data centers and supercomputers leads to the desire to further increase data rates in optical communication links. As the data rate is increased in links supported by today's optical fibers the link power budget tightens. Thus, in order to achieve reliable data signal transfers at faster data rates, one or both of source signal power must increase and signal attenuation in the optical communication link must decrease.
However, conventional low cost optical coupler designs and source devices render it problematic to increase optical signal power and to decrease attenuation in the optical path. For example, a conventional arrangement of a source, a transition lens and an optical fiber end are illustrated in FIG. 1. To maximize coupling tolerance the source 10, transition lens 20 and optical medium 30 are arranged along a shared axis 7. The transition lens 20 is arranged with a convex surface that faces the source 10 and a convex surface that faces an end face 31 of the optical medium 30. The transition lens 20 is separated from both the source 10 and the end face 31 of the optical medium by respective air gaps. These gaps create interfaces at which there is a mismatch between indexes of refraction. Consequently, such interfaces refract or redirect an incident optical signal.
As illustrated schematically in FIG. 1, the optical signal, as represented by ray 12, is emitted from the source 10 in the direction of the transition lens 20. The ray 12 is emitted at a launch angle α with respect to the common axis 7. Due to the difference in the index of refraction of air and the material used to produce the transition lens 20, when the ray 12 reaches the left-facing convex surface of the transition lens 20, the ray 12 is refracted (i.e., redirected). The redirected optical signal, represented by ray 14, is further redirected when it reaches the air gap between the right-facing convex surface of transition lens 20 and the end face 31 of the optical medium 30. A portion of the twice redirected optical signal, represented by ray 16, incident at the end face 31 enters the optical medium 30, which conveys the optical signal to an opposed end of the optical medium (not shown).
As further shown schematically in FIG. 1, a remaining portion of the twice redirected optical signal incident at the end face 31, represented by ray 32, is reflected toward transition lens 20. The reflected optical signal is redirected by the right-facing convex surface of transition lens, as represented by ray 34, and redirected again, as shown by ray 36, when ray 34 encounters the interface between the left-facing surface of transition lens 20 and air. As described in further detail below, this reflected optical signal (i.e., ray 36) can be returned to the source 10. The ray 36 is received or incident at the emitter of the source 10 at an angle of incidence β with respect to the common axis 7. As indicated in FIG. 1, the launch angle α of the emitted optical signal approximates the angle of incidence β of the reflected portion of the optical signal. As also indicated in FIG. 1 transition lens 20 is implemented with a material that exhibits a one-way signal attenuation of about 5 dB to limit the amount of reflected optical signal power that is returned to source 10.
A multi-mode optical fiber is an often preferred optical waveguide for communication links in the range of about 1 to 300 meters. Today's electronics can effectively support desired data rates in excess of about 14 Gbps using vertical cavity surface emitting lasers (VCSELs) as the light source. VCSELs are often preferred by end-users because of their high coupling efficiency with optical fibers absent a beam shaping correction as is required by other light sources.
The communication standards for such communication links call for optical fibers with flat and polished end surfaces. In use, these optical fibers reflect about 4% of the incident light energy back toward the VCSEL. As illustrated in FIG. 1, the reflected optical signal can be coupled back into the VCSEL. Over relatively short separation distances between the VCSEL and the fiber end, the reflected signal is still coherent with the emitted optical signal. It is well known that such coherent feedback can lead to destabilization of the emitted optical signal. Destabilization of the optical signal can lead to an increase in data errors. In many conventional communication links, the VCSEL produces more light energy than the link requires. To ensure stable operation of the VCSEL optical attenuation is introduced via filters or absorptive lens materials. As the reflected optical signal is attenuated twice (i.e., once on the way to the fiber end and once after the reflection), the total power reflected back to the VCSEL is usually very small. For example, for a lens with 5 dB of attenuation over a single pass the reflected power that makes it back to the VCSEL is no more than 0.4% of the emitted optical signal power.
Based on the above, an increase in optical signal power can increase the susceptibility of the VCSEL to undesirable feedback from the reflected optical signal. In addition, a reduction in attenuation in the optical path would also increase the likelihood of coupling undesirable feedback into the VCSEL.